1. Field of the Invention
The present invention relates to an ink jet recording apparatus, as well as to a method, for recording information on a recording medium by jetting a recording liquid onto the recording medium. More particularly, the present invention is concerned with an ink jet recording apparatus and an ink jet recording method which use, as the recording liquid, a blended ink formed by mixing a plurality of types of ink. The present invention is also concerned with an ink supply system.
2. Description of the Related Art
In general, conventional ink jet recording systems employ a recording head having a plurality of ink discharge openings from which ink droplets are discharged in accordance with data to be recorded towards the recording medium so as to be deposited on the latter to record the data. This type of recording system is used in, for example, printers, facsimile machines and copying machines.
Various ink discharging techniques are available for discharging the ink as the recording liquid. For instance, a heat generating element (referred to as an "electro-thermal transducer") in the ink channel near the discharge opening. In operation, an electrical signal is applied to the heat-generating element to locally heat the ink so as to generate a bubble in the ink, thereby causing a pressure change in the ink channel to discharge the ink in the form of a droplet. Another discharge technique incorporates an electro-mechanical conversion element such as a piezoelectric element.
Techniques are also known for recording halftone images or information using one of the known discharge methods described above. For instance, a method referred to as dot-density control method is known in which halftone recording is realized by controlling the density per unit area of dots of a constant size formed by the ink droplets. In another method known as dot-size control method, half-tone is realized by controlling the size of the recording dots.
Since the latter method, i.e., the dot-size control method, requires a complicated control to achieve delicate variation of dot size, the former method, i.e., dot-density recording method, is used more broadly.
Recording heads incorporating the above-mentioned electro-thermal transducers can be produced comparatively easily and can perform recording with high level of resolution by virtue of this ability to attain high dot density. In this type of recording head, however, it is rather difficult to delicately control the level of pressure generated by the electro-thermal transducer and, therefore, it is impossible to modulate the dot size in accordance with the information to be recorded. Consequently, this type of recording head cannot suitably be used in dot-size control type recording operation.
For the reasons stated above, dot-density control method is used as a primary recording method rather than the dot-size control method. The ordered dither method is known as one of the typical binary coding methods for realizing halftone image recording. This method, however, has a shortcoming in that the number of levels of tone or gradation is limited by the matrix size. That is to say, a larger of matrix is required to obtain a greater number of gradation levels. A larger matrix size undesirably increases the size of one pixel of recorded image constituted by one matrix, resulting in problems such as degradation of resolution.
A conditioned determination type dither method such as the error diffusion method also has been known as another binary coding method. The above-mentioned structural dither method is an independent determination type dither method which uses a binary-coding threshold independent of the input pixel, and the conditioned determination dither method varies the threshold level taking into consideration the values of pixels around the pixel in interest. The conditioned determination dither method, represented by error diffusion method, is advantageous in that gradation and resolution are made compatible at high levels and in that generation of unwanted moire patterns in the image recorded from a printed image is greatly reduced. This method, however, suffers from a disadvantage in that coarseness of recording dots is noticeable particularly in bright regions of the recorded image, thus degrading the quality of the image. This problem is serious particularly when the recording density is low.
In order to make the coarseness of dots less noticeable, a recording method has been proposed which employs a pair of recording heads: namely, one for discharging an ink having a low dye concentration, i.e., an ink which is of a comparatively light color and which produces an image of a comparatively low thickness on the recording medium (referred to as "thin ink", hereinafter) and one for discharging an ink having a high dye concentration, i.e., an ink which is of a comparatively dark or thick color and which produces an image of a comparatively thick image on the recording medium.
According to this multi-level tone recording method using plural types of ink having different levels of thickness of the same color, the gradation is improved particularly in the highlighted region of the recorded image when the number of levels is increased, with the result that the coarseness of dots becomes less noticeable, thus improving the image quality. This is because the highlighted portion of the image is formed by light color ink dots which are inconspicuous.
FIG. 1A is a schematic perspective view of an example of a conventional ink jet recording apparatus which relies upon the above-mentioned multi-level tone method.
Referring to FIG. 1A, a carriage 706 carries eight ink tanks 701 containing, respectively, thin and thick ink of black, cyan, magenta and yellow colors. The carriage 706 also carries a multi-head 702 having eight heads for discharging these different types of ink.
FIG. 1B illustrates the discharge openings of one of the heads on the multi-head 702, as viewed in the direction of the arrow Z, i.e., from the reverse side of the drawing sheet, of FIG. 1A.
The thick and thin inks of black, cyan, magenta and yellow are represented here by Kk, Ku, Ck, Cu, Mk, Mu, Yk and Yu. These discharge openings are arranged in parallel arrays in the direction of the arrow Y. These arrays, however, may be slightly inclined within the X-Y plane. In such a case, the discharge of inks from the respective discharge openings is conducted with predetermined time lags or delay, while the head 702 travels in the direction of the arrow X.
Referring again to FIG. 1A, a sheet feed roller 703 rotates in the direction of the arrow to cooperate with an auxiliary roller 704 in feeding a recording paper sheet 707 in the direction of the arrow y while imparting a certain level of tension to the sheet 707. Numeral 705 denotes another sheet feed roller which feeds the recording paper sheet 707 and functions to impart a certain level of tension to the recording sheet 707 in a manner similar to the roller pair 703, 704. The carriage 706 is stationed at a home position "h" illustrated by broken line when recording is not conducted and when a discharge recovery operation is being conducted. A recovery mechanism (not shown) such as capping means holds the multi-head in a predetermined condition.
The carriage 706 stationed at the home position "h" commences its movement in the direction of the arrow X along a carriage guide shaft 798, in response to a record start instruction. During the movement of the carriage, thick and thin inks of four colors are selectively discharged from the n discharge openings 801 of the multi-head 702, based on a carriage position signal which is produced by a linear encoder 709 in accordance with the travel of the carriage, whereby an image fraction of a width corresponding to the width D of the arrays of the discharge openings on the recording head. As a result of this scanning operation, ink droplets reach the recording sheets in such an order or sequence of colors that begins with the thick black ink, followed by thin black ink, thick cyan ink, light cyan ink, thick magenta ink, thin magenta ink, thick yellow ink and then by the thin yellow ink, whereby dots of these inks are formed on the recording paper sheet. When the recording is finished down to the end of the recording paper sheet, the carriage 706 is returned to the home position "h" and then again commences travelling in the direction of the arrow X. In the period between two successive passes of the head for recording, the sheet feed roller 703 rotates in the direction of the arrow so that the sheet is fed in the direction of the arrow y by a distance corresponding to the above-mentioned recording width. Thus, recording over the above-mentioned recording width and sheet feed by a distance corresponding to the recording width are executed in each reciprocating scanning motion of the carriage 706, whereby the data is recorded on the recording paper sheet.
The above-described recording method, which utilizes thick and thin inks, suffers from the following problems to be solved.
Firstly, it is to be pointed out that the described recording method requires the use of two types of ink, i.e., thick and thin inks, for each of the colors to be used. For instance, eight types of inks, as well as eight ink tanks, are needed when recording with four colors. This undesirably increases the size of the recording apparatus, and poses troublesome complications in the production and administration of the inks. In addition, the user is obliged to prepare and store a large number of ink storage and supply means such as ink tank cartridges.
The second problem is as follows.
In general, an ink jet recording apparatus cannot provide image density which is higher than that of a silver salt photograph. Therefore, when the ink jet recording apparatus is used in a copying machine or the like, it cannot reproduced with sufficient accuracy an original image if a silver salt photograph is used as the original. In recent years, performance of host computers connectable to an ink jet recording apparatus has been much improved, and there is an increasing demand for directly printing the image displayed on the display unit of the host computer. Known ink jet recording apparatuses cannot satisfactorily cope with this demand because the dynamic range of printed images printed by an ink jet recording apparatus is narrower than that of the image displayed on the computer display.
Ink jet recording apparatuses also suffer from a disadvantage in that, when a thick ink is used in order to attain a high density of recorded image, the ink discharge openings tend to be clogged with the ink which has become viscous or solidified due to evaporation of the solvent, resulting in ink discharge failure. Consequently, there is a practical limit in the increase of the ink thickness.
Thus, an ink jet recording system is required to record image with high levels of gradation and resolution within a restricted range of density, because this type of recording system can provide only a narrow range of recorded image density. This problem also is encountered in the aforementioned method which employs thick and thin inks.
It is also to be pointed out that the image quality tends to be degraded according to the composition of the solvent used in the ink.
It has been recognized that the quality of print of characters printed by an ink jet recording apparatus also varies largely depending on factors such as the type of the ink, type of the recording sheet and the combination thereof. When the ink jet recording method is used for color recording, it is necessary to use expensive special paper in order to obtain a color image of high quality having no bleeding at the boundary between regions of different colors. To obviate this problem, a study has been made in recent years to develop inks with which color printing can be done on ordinary paper sheet such as bond paper sheets or copies paper. Despite such a development, however, the quality of color printing by ink jet recording method is still unsatisfactory and further improvement in required.
Penetration and spreading of ink droplets on copies or bond paper, as well as some degree of bleeding of colors at the boundary region, can be varied by changing the composition of the solvent used in the ink.
In general, an ink which exhibits a small degree of spreading provides a high density of the dots and, hence, can suitably be used in printing sharp images such as those of characters, thin lines and so forth. This type of ink, however, exhibits only a low rate of penetration into the paper, allowing undesirable blurring at the boundary between regions of different colors. Conversely, an ink which exhibits a large degree of spreading of dots cannot produce sharp images so this type of ink is not suitable for use in printing characters and thin lines. This type of ink, however, is suitable for printing color image because of its lower tendency to blur at the boundary region.
In general, sharpness of characters and thin lines and suppression of blurring at the boundary region are incompatible with each other in the recording on an ordinary paper sheet. Namely, an ink which provides high quality of print of character exhibits a large tendency toward blurring at the boundary, whereas an ink which provides clear distinction between colors at the boundary region cannot provide high quality of print of characters.
Thus, no ink has been proposed and used which would enable recording of color images with a high degree of sharpness of characters and thin lines without substantial blur at the boundary between regions of different colors.
It would be possible to prepare two ink tanks: one containing an ink suitable for printing characters and one containing an ink suitable for recording color image, to enable the user to selectively use one of these ink tanks according to the recording purpose. Such a solution, however, inconveniently causes user confusion.
Installation of such plural ink tanks in a signal recording apparatus undesirably increases the number of the recording heeds, as well as the number of tanks and associated parts, particularly when a plurality of colors are to be used as in color recording, as is the case of the multi-color printing with thin and thick inks of each color described before.
The above-described problems are serious, especially in a recording apparatus which, in order to improve gradation, uses plural inks of different dye concentrations for each of plural colors, because such an apparatus requires troublesome work for producing and maintaining inks and ink tanks and undesirably burdens the users due to the necessity of preparing many ink tanks containing different types of inks.